COROCO-CLAVICULAR UNION 251 



(a) The Acromio-clavicular Joint 

 Class. — Diarthrosis. Subdivision. — Arthrodia. 



The acromio-clavicular joint is surrounded by an articular capsule and fre- 

 quently contains an articular disc. 



The articular capsule (figs. 287 and 290) completely surrounds the articular 

 margins, and is composed of strong, coarse fibres arranged in parallel fasciculi, 

 of fairly uniform thickness, which are attached to the borders as well as the 

 surfaces of the bones. It is somewhat lax in all positions of the joint, so that the 

 clavicle is not tightly braced to the acromion. The fibres extend three-quarters 

 of an inch (2 cm.) along the clavicle posteriorly, but only a quarter of an inch 

 (6 mm.) anteriorly. Superiorly, they are attached to an oblique line joining 

 these two points, while inferiorly they reach to the ridge for the trapezoid liga- 

 ment with which they blend. 



At the acromion they extend half way across the upper and lower surfaces, but at the 

 anterior and posterior limits of the joint they are attached close to the articular facet. The 

 anterior fibres become blended with the insertion of the coraco-acromial ligament. The 

 fibres are strengthened above by the aponeuroses of the trapezius and deltoid muscles; and all 

 run from the acromion to the clavicle medially and backward. 



The articular disc is occasionally present, but is usually imperfect, only oc- 

 cupying the upper part of the joint; it may completely divide the joint into two 

 cavities, or be perforated in the centre. It is usually thicker at the edge than 

 in the centre, and some of the fibres of the articular capsule are blended with its 

 edges. 



The synovial membrane lining the joint is occasionally either partially or 

 entirely divided into two by the articular disc. 



Relations. — Superiorly skin and fascia and the tendinous intersection between the deltoid 

 and the trapezius. Inferiorly, the coraco-acromial ligament and supraspinatus. Anteriorly, 

 part of the origin of the deltoid. Posteriorly, part of the insertion of the trapezius. 



Movements. — A certain amount of gliding movement occurs at this joint, but the most 

 important movement is a rotation of the scapula whereby the glenoid cavity is turned forward 

 and upward, or downward. As these movements occur the inferior angle of the scapula moves 

 forward as the glenoid cavity turns upward and the superior angle recedes. 



The forward movement of the inferior angle is produced mainly by the inferior fibres of 

 the serratus anterior (magnus), aided by the inferior fibres of the trapezius, and it is by this 

 movement that the arm is raised above the level of the shoulder forward. 



The reverse movement is produced mainly by the rhomboideus major aided by the latissimus 

 dorsi. 



(h) The Coraco-clavicular Union 



The coraco-clavicular ligament (figs. 286, 287, and 290) consists of two parts, 

 the conoid and the trapezoid ligaments. 



The conoid ligament is the medial and posterior portion, and passes upward 

 and laterally from the coracoid process to the clavicle. 



It is a very strong and coarsely fasciculated band of triangular shape, the apex being 

 fixed to the medial and posterior edge of the root of the coracoid process just in front of the 

 scapular notch, some fibres joining the transverse ligament. Its base is at the clavicle, where 

 it widens out, to be attached to the posterior edge of the inferior surface, as well as to the cora- 

 coid tubercle. It is easily separated from the trapezoid, without being absolutely distinct. A 

 small bursa often exists between it and the coracoid process; medially, some of the fibres of the 

 subclavius muscle are often attached to it. 



The trapezoid ligament is the anterior and lateral portion of the coraco- 

 clavicular ligament. It is a strong, flat, quadrilateral plane of closely woven 

 fibres, the surfaces of which look upward and medially toward the clavicle, and 

 downward and laterally over the upper surface of the coracoid process. 



At the coracoid it is attached for about an inch (2.5 cm.) to a rough ridge which runs 

 forward from the angle, along the anterior border of the process. At the clavicle it is attached 

 to the obhque ridge which runs laterally and forward from the coracoid tubercle, reaching as 

 far as, and blending with the inferior part of the acromio-clavicular ligament. Its anterior 

 edge is free, and overlies the coraco-acromial ligament; the posterior edge is shorter than the 

 anterior, and is in contact with the posterior and lateral portion of the conoid hgament. 



The arterial supply is derived from the transverse scapular (suprascapular), acromial 

 branches of the thoraco-acromial, and the anterior circumflex. 



The nerve-supply is derived from the suprascapular and axillary (circumflex) nerves. 



